Hydraulic elevator control systems

ABSTRACT

HYDRAULIC ELEVATOR CONTROL SYSTEM MAINTAINS DOWNSPEED ESSENTIALLY CONSTANT FOR ALL ELEVATOR LOADS. DEVICE POSITIVELY RESPONSIVE TO FLUID FLOW THROUGH DOWN-CONTROL VALVE CONTROLS A METERING VALVE TO REGULATE FLUID FLOW IN A DOWN-CONTROL VALVE BLEED LINE TO REGULATE THE DEGREE OF OPENING OF THE DOWN-CONTROL VALVE MAIN FLOW APERTURE.

Nov. 9., 1971 E. E. .JOHNSTON Re- 27,233

HYDRAULIC ELEvAToR CONTROL SYSTEMS Original Filed April l0 2` Sheets-Sheet 1 ORNEYS Nov. 9, 1971 Original Filed HYDRAULIC ELEVATOR CONTROL April 1o 1967 E. E. JOHNSTON Re. 27,233

SYSTEMS 2 Sheets-Sheet 2 [Ply/.5 RESERVOIR f2,

l A i 9'" 213" CAR PUMP SV-4 N-4 Y u /27 n FLOOR /5 5l 29 B 1 UP CONTROL fzs A BY-PASS f Y VALVE zo .37 N-S SV-3 SV-S j T FLOOR s PILOT J9 A VALVE f/J Y CHECK fig L1 VALVE MLV .55 2l 53 ,25 |/4J 2a Z1X FLOW DOWN RESPONSIVE CONTROL n VALVE VALVE N 5 l 4J 45 t V l t SV| SV-2 T15 V 57 L .1 V E47 5 JACK i N l NH2 I7 4% l i METERlNG I l VALVE CAR CONTROLLER PUMP MOTOR SV-l SV-2 SV-S SV-4 SV-5 2H CONTROLLER (NC.) (N C.) (NC.) (No.) (No.)

CAM SELECTOR CONTROLLER A k I 7 INVENTOR. 00 W1/Jeff z3.' JJ/msfow TTORNEYS United States Patent Oce Re. 27,233 Reissued Nov. 9, 1971 27,233 HYDRAULIC ELEVATR CONTROL SYSTEMS Everett E. Johnston, Newark, Tex., assignor to Esco Elevators, Inc., Fort Worth, Tex.

Original No. 3,438,398, dated Apr. 15, 1969,- Ser. No. 629,520, Apr. 10, 1967. Application for reissue Apr. 13, 1970, Ser. No. 28,158

Int. Cl. B66b 1/24, 5/04 U.S. Cl. 137-608 2 Claims Matter enclosed in heavy brackets appears in the original patent but forms no part of this reissue specilication; matter printed in italics indicates the additions made by reissue.

ABSTRACT F THE DISCLOSURE Hydraulic elevator control system maintains downspeed essentially constant for all elevator loads. Device positively responsive to iluid ow through down-control valve controls a metering valve to regulate uid flow in a down-control valve bleed line to regulate the degree of opening of the down-control valve main ow aperture.

Background of the invention For many years it was common to permit a large variation in hydraulic elevator no-load to fullaload downspeed. More recently, increased hydraulic elevator speeds have made it highly desirable to maintain down-speed as nearly constant as possible for all loads. Hydraulic elevator down-speed control in accordance with the present invention achieves improved accuracy as to the ideal of constant down-speed for all loads, accompanied by improved simplicity and economy.

Brief description of drawings FIG. l is a schematic side elevational view of a downcontrol valve, ilow responsive valve, and metering valve that may be used in a hydraulic elevator control system in accordance with a preferred embodiment of the invention; FIG. 2 is a top plan view of the apparatus of FIG. l; FIG. 3 is a longitudinal vertical sectional view taken along lines III-III of FlG. 2, but with solenoid valves and associated manifold assembly removed; FIG. 4 is an enlarged plan View of the metering valve metering head; FIG. 5 is a schematic diagram of the fluid system of a hydraulic elevator in accordance with a preferred embodiment of the invention; FIG. 6 is a schematic diagram of electrical controls which pertain to the systern of FIG. 5; and FIG. 7 is an enlarged fragmentary vertical section view showing portions of the metering valve.

Description of Preferred embodiments Referring now to the drawings and initially to FIG. 5 for an overall description of the apparatus, the numeral 11 designates a uid reservoir which is preferably located in an elevated position with respect to a fluid pump 13, normally operated by an electric motor (not shown). A primary conduit 15 connects the pump 13 with a hydraulic jack 17. A `check valve 19 is interposed in the primary conduit 15. A return conduit 21 communicates with the primary conduit 15 on the down stream side of the check valve 19, and via a down control valve 23 and a ow responsive valve 25 back to the reservoir 11. An up control lay-pass conduit 28 communicates with the primary conduit 15 on the up stream side of the check valve, 19 and via an up control by-pass valve 27 to the return line 21 on the down stream side of the ilow, responsive valve 25. The up control by-pass valve 27 has a control port 29. A dump line 31 -communicates between the control port 29 and the return conduit 21. A solenoid valve designated SV-4 is interposed in the dump line 31. A slow down line 33 communicates between the control port 29 and the return conduit 21. A slow down orce or needle valve N-4 is interposed in the slow down line 33. An up acceleration line 35 communicates between the primary conduit on the up stream side of the check valve 19 and the control port 29. A needle valve N-3 and a solenoid valve SV-3 are interposed in the up acceleration line 35. An up leveling line 37 communicates between the control port 29 andthe primary conduit 15 via check valve 19. A solenoid valve SV-S and a pilot valve 39 are interposed in the up leveling line 37.

rl`he down control valve 23 has a down speed port 41, a down deceleration port 43 and a down leveling port 45. A down speed line 47 comunicates between the down speed port 41 and the return conduit 21. A solenoid valve SV-1, a needle valve N-l and a metering valve 49 are interposed in the down speed line 47. A down leveling line 51 communicates between the down leveling port 45 and the return conduit 21. A solenoid valve SV2, a needle valve N-Z and the metering valve 49 are interposed in the down leveling line 51. A down deceleration line 53 communicates between the down deceleration port 4-3 and the down speed port 41. A needle valve N-5 is interposed in the down deceleration line 53. A manual lowering line 55 communicates between the down deceleration port 4-3 `and the return conduit 21. A manual lowering valve MLV is interposed in the manual lowering line 55. The metering valve 49 is mechanically related to the flow responsive valve 25 as indicated by the dotted line 57.

Although a complete hydraulic elevator control system has been shown in FIG. 5, the present invention is concerned only with the down-control portion of the systern. Accordingly, details of the up-control portion of the system are not described or shown herein. Actually, various up-control systems could be used in hydraulic elevator control systems embodying the present invention. For further details of the up-control portion of the control system of FIG. 5, reference is made to U.S. Patent No. 3,266,382.

Referring now particularly to FIGS. l, 2 and 3 of the drawings, there is shown a down-control valve 23, a flow responsive valve 25 and a metering valve 49 typical of those which may be utilized in the practice of the present invention. The down-control valve 23 includes a valve body 65 having an inlet passage 67 leading to an inlet cavity 69, and an outlet cavity 73. Interposed between the inlet and outlet cavities 69, 73 is a Iwall structure 75 having an opening therein which is the main valve aperture 77. Circumferentially disposed about the main valve aperture on the inlet side is a main valve seat 79. The valve body 65 also has a cylindrical control cavity 81 disposed coaxially with the main valve aperture 77 and opening on the side of the inlet cavity 69 opposite the main valve aperture 77.

One end portion of a spool member 83 forms a piston which works in said cylindrical control cavity 81, nd the other end portion forms a valve disc 87. The pool member 83 has a central bore 89 which at one nd portion receives the cylindrical stern 19 of a spool uide 93. The spool guide 93 is secured to the spool member 83 by means of a set screw 95 which engages is stem 91. A gasket 97 is sandwiched between the end ace of the valve disc 87 and the closed end of the spool uide 93, and has an exposed annular face which engages he main valve seat 79 when the main valve aperture 77 i closed. The spool guide 93 'has a cylindrical portion 9 which makes a sliding tit with the main valve aperure 77 so as to provide guide support for the valve isc end of the spool member 83. The spool guide 93 Las a reentrant cup-shaped cavity 101 co-axial with the tem portion and opening on the outlet cavity 73. The vfall of the spool guide cylindrical Vportion 99 is slotted D provide lluid flow apertures communicating between he inlet and outlet cavities and of size depending on the x-ial position of the spool member 83.

The valve body 65 has a threaded bore 103 disposed o-axially with the cylindrical cavity 81 and leading from he outer end of same to the valve body exterior. A rarrel member 105i has a threaded exterior por-tion ngaging the valve body threaded bore. The inner end lf the barrel member terminates in a llange '107 which erves as a stopI toy limit the outward travel of the piston I5. A lock nut 109 engages the barrel threaded exterior o -x the flange stop 107 at a selected position. The Iarrel member 105 has a cylindrical bore 111 extending rom the dlange stop end and merging with a smaller hreaded bore. A stem member 113 has exterior threads vhich engage those of said smaller threaded bore and nerge at one end with a guide piston 115 which engages aid barrel member cylindrical bore 111. The stem memer 113 has a cylindrical outer end portion with a cirumferential Igroove disposed intermediate its ends. A fitting 117 has a barrel portion which makes a sliding it on said stern member 113, and with said groove, orms an annulus t119 which communicates with a pasage to the down-leveling port 45. Suitable seals are rovided on both sides of the annulus between the T ittin-g barrel portion and the stem member. A lock nut .21 engages the stem member threaded portion and ecures the stem member 113 at the selected axial posilon. A keeper ring 123 retains the T fitting barrel porion on the stern member 113. The stern member 113 has tn axial bore 125 communicating at one end, via radial penings with the annulus 119 and at the other end Vith a threaded bore which receives the threaded end of t cylindrical extension member 127. The extension memaer 127 has an axial bore which mates with and proyides an extension of the stem member bore 125. The :nd of the extension member 127 opposite the threaded :nd is received by the central bore 89 of the spool nember 83, and the extension member bore terminates rear this end where it communicates with a radial orice mening, which for convenience will be herein referred o as the down leveling orifice 129.

The valve body 65' has a passage leading from the lown speed port 41 to the cylindrical control cavity 81 lt the outer end portion thereof. The valve body 65 ilso has a passage leading from the down deceleration Jort 43 to the inlet cavity 69. The down control valve las a manifold assembly 133 which mounts the solenoid valves S-V1 and S-V-Z and needle valve N-S. The manifold assembly 133 has suitable internal passages to procgmmunication to effect the connections shown by The flow responsive valve 25 and the metering valve 49 are carried by a support body 13S of generally cylindrical shape and 'having a flanged end 137 secured by bolts 139 to the down control valve body 65. 'Bhe support body 135 has a central bore 141 coaxial with an outlet aperture 71 of the outlet cavity 73. Disposed within said support body central bore is a web structure 143 having a central bore 145 coaxial with the outlet aperture 711. The web structure 143 has axially extending openings 147 to permit free fluid flow. 'Ilhe iiow responsive valve 25 includes a head portion 149, a guide sleeve 151 and a control cone 153. The head portion is generally cup-shaped, with the cup sidewall exterior being cylindrical and making a sliding fit with said outlet aperture 71. The cup bottom lhas an axial bore 155 and also an outwardly extending circumferential lflange 157 having an annular side face that can seat on the annular exterior surface of the outlet cavity end wall 159 adjacent the outlet aperture 71. The guide sleeve 151 makes a sliding lit with the web structure central bore 145. The guide sleeve 151 is disposed between the bottom of the head portion 149 and the control cone 153. A bolt rod 1611 passes through and slidably engages the bores of the head portion 149, guide sleeve 151, and control cone 153. Nut-s 163 engage threaded ends of the bolt rod 161 to clamp the lhead portion, sleeve, and control cone together. A compression spring 165 surrounds the guide sleeve '151 and urges the head portion 149 toward the closed position. The head portion sidewall is slotted to provide -ufid ilow apertures communicating between the outlet cavity 73 and the support body central bore 141 and of size depending on the axial position of the llow responsive valve 25.

On the support body 135 near the downstream side of the web structure 143 is a boss 167 having a threaded bore the central axis of which intersects the central axis of the support body. The metering valve 49 includes a barrel portion 169, a stem portion 171, a plunger 173, and a metering member 175. The barrel portion outer end is in the form of a bolt head 177. The barrel exterior from the bolt head to the inner end is threaded to mate with the boss threaded bore. A lock nut 179 threadedly engages the barrel portion to tix the barrel portion in the selected axial position. The barrel portion 169 has a cylindrical bore 181 which extends from an internally threaded bore l183 at the barrel outer end to an inwardly extending flange 185 at the barrel inner end. The stem portion 171 has `a bolt head 1187, external threads adjacent the bolt head and engaging the internally threaded bore 183 of the barrel portion 169, an intermediate cylindrical portion of reduced diameter adjacent the external threads, and a lower cylindrical portion of further reduced diameter adjacent the intermediate cylindrical portion. The stem portion 171 also has an internally theaded bore 189 opening to a central bore 191. The plunger 173 is cup-shaped, having an exterior cylindrical surface that makes a sliding fit with the barrel portion cylindrical bore 181, and having an interior cylindrical surface that makes a sliding lit with the lower cylindrical portion of the stem 171. The cup bottom 193 has an axial opening 195. The metering member has a head 197 and a positioner arm 199. The head is generally disc-shaped and is disposed within the barrel cylindrical bore 181 between the plunger 173 and the inwardly extending flange 185. The positioner arm 199 is integral with and :depending from the head 197, and has its lower end positioned to coact with the control cone 153 of the ow responsive valve 25. The -head upper surface includes an annular groove 201. A plurality of orice passages 203 extend from the bottom of the groove 201 through the head 197. A compression spring 205 encircles the intermediate cylindrical portion of the stem '171 and urges the plunger 173 downwardly into contact with the head 197.

The electrical controls illustrated by the schematic diagram of FIG. 6 include a car controller 207, a cam selector controller 209, a pump motor controller 211, and solenoid valves SV-1 through ASV-S. The pertinent function of these controls will become apparent from the subsequent description of operation of the down-s control portion of the hydraulic elevator control system of FIG. 5.

Assume that the elevator car 213 is at rest at floor B. The car and its load are supported by the hydraulic uid in the jack 17, in the primary conduit between the jack and the check valve 19 which is closed, and in the return conduit 21 between the primary conduit and the down-control valve 23 which is closed. Solenoid valve `SV--l and SV-Z are also closed.

To start downward, the down button (on car controller 207) is depressed, causing solenoid valve SV-1 to open. This permits fiuid flow from the control cavity 81 via down speed port 41, SV-1, down speed line 47, needle valve N-1, metering valve 49, and return line 21, to the reservoir y11. Pressure force differential on the piston 85 will now move the piston `85 (and consequently the valve disc 87) toward the stop flange 107 thus opening the down control valve 23, permitting fluid flow from the inlet cavity 69 via the main valve aperture 77 to the outletfcavity 73. This, of course, permits uid ow from'th'e jack 17 via the down-control valve 23 and the ow responsive valve 25 to the reservoir 11. As soon as Huid starts flowing from the jack, the elevator car 213 bings to descend. The rate of acceleration of the car to full down speed is determined by the setting of the needle valve N-1.

The piston 85 will stop and remain at a position corresponding to the elevator car fulldown speed and a given load, as will hereinafter be explained. While the elevator car 213 is descending at full down speed, the down levelling orifice 129 is covered by the spool member central bore `89. When the car 213 has reached a certain position between floors A and B, a cam on the cam selector controller 209 actuates a switch which energizes solenoid valve SV-2, which then opens. Then another cam on the cam selector controller 209 actuates a switch which de-energizes solenoid valve SV-l, which then closes. Since closing of SV-l stops the bleeding of fiuid from the control cavity 81, a pressure build up on control cavity side of the piston 85 will cause it to move toward the main valve aperture closing position at a rate determined by the setting of the down deceleration needle valve N-S. When the piston 85 has moved sufficiently to uncover the down levelling orifice '129, fluid will bleed via the down levelling port 45 and down levelling line 51 and metering valve 49 to the reservoir 11, at a rate sufiicient to stop the closing movement of the piston 85. The regulating action of the down levelling orifice 129 will maintain the piston 85 at the levelling speed position. The down levelling speed is predetedmined by adjustment of the stern member `113 to place the orifice 129 at a selected axial position.

At the moment the elevator car reaches floor A, another cam on the cam selector controller 209 actuates a switch which de-energizes solenoid valve SV-Z, which then closes. The closing of SV-2 produces an immediate buildup of pressure on the control cavity side of the piston 85 and the down control valve 23 closes to stop the car 213.

As hereinbefore stated, the present invention is directed to a hydraulic elevator down control system wherein the elevator car full down-speed is maintained as nearly constant as possible, regardless of load. The operation as to the down-speed control will now be explained.

In order for the elevator car 213 to descend at a selected constant down-speed, the rate of fluid flow from the jack 17 via the down-speed, the rate of fluid ow aperture 77 to the reservoir 11 must be constant. For a given elevator load there is a position of the spool member piston 85 that will provide the proper opening at the main valve aperture 77 to permit the ow rate that is required' for the selected down-speed. When the elevator load is changed, the flow rate for a given position of the piston 85 (and consequently a given opening at the main valve aperture 77) changes because the fluid pressure on the jack side of the main valve aperture 77 changes. Therefore, when the elevator load changes, the position of the piston must change (to change the opening at the main valve aperture 77) if the flow rate is to be maintained constant. The down-speed control operates to move the piston 85 to the position required to produce the selected flow rate (and consequently the selected down-speed) at the elevator load (pressure within the inlet cavity 69) that at that moment exists, and to maintain the piston 85 in that position.

Fluid pressure in the outlet cavity 73 upon opening of the down control valve 23 will cause the flow responsive device 25 to move in the downstream direction. The fluid ow through the outlet operture 711 will produce a pressure differential on the head portion 149 of the ffow responsive device 25 which will urge the head portion in the downstream direction against the force ofthe compression spring 165. The axial position of the flow responsive valve, and consequently the position of the cone 153 and positioner arm 199, is determined by the pressure differential on the head portion 149, which in turn is determined by the rate of fiuid ow through the outlet aperture 71. The position of the anni 199 determines the position of the head 197 Iwhich determines the degree of opening of the metering valve 49.

Assume that there is no load on the elevator car 213 and that it is accelerating toward full down-speed. Fluid will be flowing into the control cavity y81 from the inlet cavity `69 via ports 43 and 41 and line 53, and at the same time fluid will be bleeding away from the control cavity via line 47 and the metering Valve 49. The metering valve 49 will be closing since the fiow rate at the outlet aperture 71 will be increasing as the down control valve 23 opens. As the metering valve 49 closes, the pressure force differential on the piston 85 decreases, and when the metering valve 49 has closed to the position where the pressure force differential on the piston 85 is zero, the piston stops moving. The position of the control cone y153 relative to the positioner arm 199 has been pre-set such that the piston 85 stops at the position corresponding to that opening of the down control valve 23 at the main valve aperture 77 which will permit that flow rate required to establish the selected down-speed at the no load condition.

Assume now that there is full load on the elevator car 213 and that it is accelerating toward full downspeed. The action under this condition is similar to that described above with reference to the no load condition. However, due to the greater load, the pressure in the inlet cavity 69 will be greater; the initial differential pressure force on the piston 85 will be greater; the flow rate through the metering valve 49 will be greater; and the closing rate of the metering valve will be greater. The consequence of these new parameters is that the differential pressure force on the piston 85 will reach Zero before the piston has travelled as far as in the no load case, with the piston stopping at the position correspond ing to the opening of the down control valve 23 at the main valve aperture 77 which will permit that ow rate required to establish the selected down-speed. The selected down-speed is of course the same for the no load and the full load conditions. Also, the ow rate past the main valve aperture 77 is the same for the no load and full load conditions. The difference is that the opening at the main valve aperture 77 is smaller for the full load condition than for the no load condition. The pressure drop across the outlet aperture 71, and consequently the degree of opening of the metering valve 49, is the same for the full load and no load conditions.

For any load between no load and full load, the action is similar to that just described for the no load and full load conditions. However, for each load condition the differential pressure force on the piston 85 will reach zero when the piston 85 is in a position corresponding to the opening at the main valve aperture 77 which will aermit that iioW rate required to establish the selected town-speed.

Any increase or decrease in pressure in the inlet cavity i9 will cause the piston 85 to immediately assume a new )alance position corresponding to the opening at the main 'alve aperture 77 which will permit that ow rate required o establish the selected down-speed. The term elevator oad as used herein, in addition to the actual load carried y the elevator, includes any factor that will cause an inrease or decrease in pressure in the inlet cavity. For exrnple, an elevator jack of the plunger type will exhibit a .oating effect which varies with the distance of the ilunger from the bottom of the jack cylinder, and this vill affect the inlet cavity pressure and will consequently ary the elevator load as the elevator descends. Also for Xample, an elevator jack of the plunger type will exhibit 'ariations in `friction between the plunger and its hydrauic seals over the plunger length, and this will affect the nlet cavity pressure and will consequently vary the :levator load. The response of the down-speed control ystem of the instant invention to variations in pressure n the inlet cavity is such that there is no noticeable variaion in the elevator car down-speed.

Briey stated, the present invention contemplates a xydraulic elevator down-control system of the type wheren a primary conduit connects a fluid reservoir via a pump rnd a check valve to a jack, and a return conduit is connected to the primary conduit on the jack side of the :heck valve and communicates via a down control valve Vith the reservoir. The down control valve would have a nain valve aperture interposed between an inlet cavity .nd an outlet cavity and closure means for determining he opening at the main valve aperture. There Iwould be rovided means defining a control cavity, a piston recipocable within the control cavity, with the piston having t rst side and a second side. There would be provided neans coupling the piston with the main valve aperture :losure means so that the position of the closure means rnd consequently the opening at the main valve aperture vould be determined by the position of the piston. There vould be provided means permitting iluid from the inlet :avity to exert pressure on both sides of the piston and neans providing a greater area exposed to liuid pressure )n the rst side of the piston than on the second side. A low responsive device would be interposed in the return :onduit at a region intermediate the down-control valve rnd the reservoir, with the iiow responsive device having L movable element thevposition of which is directly conrolled by the flow rate of fluid passing through the device. [here would be provided conduit means communicating )etween the control cavity on the piston rst side and the 'eturn line, with a metering valve interposed in the conduit neans and coupled to the flow responsive device movable :lement so that movement of the movable element to pernit greater flow rate of uid passing through the flow esponsive device will move the metering valve toward the :losed position and movement of the movable element o permit lesser How rate of uid passing through the low responsive device will move the metering valve tovard the opened position.

The term jack as used herein encompasses any power :ylinder means wherein the position of a piston or plunger Nithin a cylinder is controlled by hydraulic fluid and )ower is transmitted from the power cylinder means to 1n elevator car.

While I have shown my invention in only one form, t will be obvious to those skilled in the art that it is not ao limited, but is susceptible of various changes and modiications Without departing from the spirit thereof.

I claim:

1, In a hydraulic elevator down-control system of the :ype having a primary conduit that connects a fluid `eservoir via a pump and a check valve to a jack, a return :onduit connected to said primary conduit on the jack side nf said check valve and communicating via a down control valve with said reservoir, with said down control valve having a main valve aperture interposed between an inlet cavity and an outlet cavity and closure means for determining the opening at said aperture, the improvement comprising:

(a) means defining a control cavity;

(b) a piston reciprocable within said control cavity and having a iirst side and a second side;

(c) means coupling s aid piston with said closure means so that the position of said closure means and consequently the opening at said aperture is determined by the position of said piston;

(d) means permitting fluid from said inlet cavity to exert pressure on both sides of said piston;

(e) means providing a greater area on the rst side of said piston exposed to liuid pressure than on the second side of said piston;

(f) a flow responsive device interposed in said return conduit at a region intermediate said down control valve and said reservoir, said flow responsive device having a movable element the position of which is directly controlled by the flow rate of fluid passing through said device;

(g) conduit means communicating between said control cavity on said piston rst side and said return line; and

(h) a metering valve interposed in said conduit means and coupled to said ow responsive device movable element so that movement of said movable element to permit greater ow rate of fluid passing through said device will move said metering valve toward the closed position and movement of said movable element to permit lesser ilow rate of uid passing through said device will move said metering valve toward the opened position.

2. In a hydraulic elevator down-control system of the type having a primary conduit that connects a fluid reservoir via a pump and check valve to a jack, a return conduit connected to said primary conduit on the jack side of said check valve and communicating via a down control valve with said reservoir, with said downy control valve having a main valve aperture interposed between an inlet cavity and an' outlet cav-ity and closure means for determining the opening at said aperture, the improvement comprising:

(a) means defining a control cavity;

(b) a piston reciprocable within said control cavity and having a first side and a second side;

(c) means coupling said piston with said closure means so that the position of said closure means and consequently the opening at said aperture is determined by the position of said piston;

(d) means permitting fluid from side inlet cavity to exert pressure on both sides of said piston;

(e) means providing a greater area on the first side of said piston exposed to fluid pressure than on the second side of said piston;

(f) a flow responsive device interposed in said return conduit at a region intermediate said down control valve and said reservoir, said iiow responsive device having a movable element the position of which is directly controlled by the flow rate of fluid passing y through said device;

9 10 through said device will adjust said orifice toward the 3,037,354 6/ 1962 Tennis 137-6\12.1 X Opened position- 3,125,319 3/1964 Afbogastet a1 91-461 X References Cited 11%)/ Ladef '"'"""`379 The following references, cited by the Examiner, are of 5 2 Du u record in the patented le of this patent or the original SAMUEL SCOTT Primary Examiner patent.

UNITED STATES PATENTS U.S. Cl. X.R. 2,600,702 6/1952 Stephens 137-608 91-461 2,911,006 11/1959 vogel 137-608 X 10 

